For economic reasons, reaction rate and energy efficiency of ultrasonic devices need to be higher. In this investigation, it was found that the 850 kHz ultrasonic device achieves a higher energy efficiency and a higher sonochemical reaction rate than the 24 or 30 kHz device during the oxidation of KI or the degradation of benzene in aqueous solutions. Moreover, lower power intensity or higher power density is generally favorable to the oxidation of KI. For benzene degradation, however, energy efficiency is enhanced with the increase in power intensity and density at lower input power and decreases with the increase in power intensity and density at higher input power. In addition, energy efficiency can also be diminished by changing fluid motion, for example, shortening the distance between the tip of the ultrasonic horn and the reactor bottom, and stirring during sonication.